In any system employing a natural language interface, there is the problem that, by means of a formal grammar, the system itself defines the language it will accept. But, when using language, people will not always adhere to the rules of this grammar; therefore, a natural language computer system should not simply treat as incomprehensible any input not conforming to its internal grammar, input we may call extragrammatical. The term extragrammatical refers to input that is not necessarily incorrect in an absolute sense but only relative to the formal scope of a system's grammar. Before a truly robust system can be developed, what is needed is a parsing mechanism that enforces grammaticality where possible, and this implies a deterministic approach to natural language parsing. This thesis discusses the importance of flexible natural language interfaces; the notion of extragrammatical language and its connexion to robust parsing; a deterministic parser, PARSIFAL, developed by Mitchell Marcus; and a reimplementation, using logic programming, of a subset of Marcus' system. Programming was done with CProlog on a VAX 11/750* running 4.2 BSD UNIX.

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